1. Field of the Invention
The invention relates to the field of ultrasonic flowmeters and, in particular, to an ultrasonic flowmeter that combines flow conditioning technology with simple and inexpensive ultrasonic technology to yield high accuracy. The method and apparatus of the invention does not require the use of integration techniques or the prior determination of flow swirl or asymmetry to achieve accuracy. The invention also allows implementation of various self-diagnostic features.
2. Description of the Prior Art
Referring now to FIG. 1, flowmeters are generally classified as either energy additive or energy extractive. Energy additive meters introduce energy into the flowing stream to determine flowrate. Common examples of energy additive meters are magnetic meters and ultrasonic meters. Energy extractive meters require energy from the flowing stream, usually in the form of pressure drop, to determine the fluid's flowrate. Examples of energy extractive meters are PD meters, turbine meters, vortex meters and head meters (orifice, pitot, venturi, etc.).
Further subclasses of flowmeters are based on determining if the meter is discrete or inferential. Discrete meters determine the flowrate by continuously separating a flow stream into discrete segments and counting them. Inferential meters infer flowrate by measuring some dynamic property of the flowing stream.
Ultrasonic flowmeters are energy additive inferential flowmeters. They are well known in the art and can be further subclassified as shown in FIG. 2. Ultrasonic flowmeters determine the velocity of the flowing stream from the difference in transit time of acoustic pulses transmitted in the downstream and upstream directions between acoustic transducers. These acoustic pulses are transmitted along a chordal path, and a measure of the average chordal velocity is determined from the measured transit times. The fluid can be gas or liquid.
Transit times depend on the mean velocity of the chordal path, the flow profile and the turbulence structure of the flowing stream. The reliability of the measured chordal velocity depends on the path length, the configuration and radial position of the acoustic path, the transmitted acoustic pulse form, the electronic timing and gating performance and the calculations involved in reducing the measured parameters to the mean chordal velocity.
Acoustic transducers can be mounted in an invasive or non-invasive manner. An invasive mount invades the channel's containment structure through an aperture and allows the transducer to transmit acoustic pulses directly into the flowing stream. Invasively mounted transducers are also referred to as “wetted” transducers. A non-invasive mount transmits the acoustic pulses through all or part of the channel's containment structure. Transducers mounted in this fashion are also referred to as “non-wetted” transducers.
The invasive mount is further classified as intrusive or non-intrusive. Intrusive mounting means that all or part of a transducer intrudes into the flowing stream. Non-intrusive mounting means that the transducer is recessed and does not intrude into the flowing stream.
Acoustic paths may be arranged in a reflective, non-reflective or hybrid geometry. A reflective path is arranged in a geometric manner to reflect one or more times off the containment structure or reflective bodies installed inside the channel. A non-reflective path is arranged in a geometric manner that does not reflect off the containment structure or a reflective body inside the channel. A hybrid is a design that employs both reflective and non-reflective paths. The number of paths and their placement in the channel vary among state of the art designs.
Ultrasonic flowmeters have been the center of attention within the natural gas industry for the last decade. State of the art ultrasonic flowmeters employ one of two commercially available integration methods to determine the average flow velocity in a circular duct. A third integration method is under development by the scientific community. Both commercial methods perform well in the laboratory environment of “fully developed” pipe flow. However, in the industrial environment, multiple piping configurations assembled in series generate complex problems for flow-metering engineers. The challenge is to minimize the difference, i.e. achieve “similarity,” between the actual, field flow conditions and laboratory, “fully developed” flow conditions. The correlating parameters which impact similarity vary with meter type and design. However, it is generally accepted that the level of sensitivity to time-averaged velocity profile, turbulence structure, and bulk swirl is dependent on the metering technology and the specific design of that meter.
The first integration method, known as Gaussian integration, is based on a fixed number of paths whose locations and correction factors are based on the numerical Gaussian method selected by the designer. Several Gaussian methods are available from publications (Jacobi & Gauss, Pannell & Evans, etc.) or disclosed in U.S. patents such as U.S. Pat. Nos. 3,564,912, 3,940,985, and 4,317,178. The advantages of this approach are clear. No additional information of the flow profile is required for calculating the average flowing velocity. The correction factors are fixed in advance as a result of the number of paths and the Gaussian method selected by the designer. Gaussian integration methods require at least four paths to yield acceptable results. Based on available public research, Gaussian integration methods have a bias uncertainty of up to 3% due to variations in piping configuration.
The second integration method, disclosed in U.S. Pat. No. 5,546,812, determines the swirl and asymmetry of the flowing stream by transmitting acoustic pulses along two or more paths having different degrees of sensitivity to swirl and to symmetry. This method uses a conversion matrix to determine the correction factors for the chordal velocities based on the measured swirl and asymmetry. The recommended number of paths is five for the proprietary method. According to available literature, this integration method has an additional bias uncertainty of up to 1% due to variations in piping configuration.
The third integration method, now under development by the National Institute of Standards and Technology (NIST) is an eleven-path arrangement. The unit, termed the advanced ultrasonic flowmeter (AUFM), is based on computer modeling of pipe flow fields and simulations of their corresponding ultrasonic signatures. The sensor arrangement for the AUFM will have enhanced velocity profile diagnostic capabilities for deviations from non-ideal pipe flows. A pattern recognition system capable of classifying the approaching unknown flow among one of a number of typical flows contained in an onboard, electronic library will interpret the acoustic signals. The flow library will be created using results from computational fluid dynamics simulations. No bias uncertainty information is currently available for this experimental integration technique.
All of the state of the art ultrasonic flowmeters suffer from the disadvantage of high cost due to the requirement of at least four paths (up to eleven paths in the AUFM). Each path requires a pair of transducers with associated mounting mechanisms and wiring. Thus current ultrasonic flowmeters are costly and maintenance intensive. In addition, under real-world industrial conditions, current ultrasonic flowmeters suffer relatively high bias uncertainty errors due to swirl and asymmetry effects. These disadvantages are overcome by the present invention.